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opcua-session-manager · v4.3.x
Docs · IPC

IPC overview

NDJSON-framed flat JSON envelopes over a local socket. Two transports, one wire format, seven commands. The whole protocol fits on one screen.

The IPC channel between ManagedClient and SessionManagerDaemon is intentionally narrow:

  • Transport. One of UnixSocketTransport or TcpLoopbackTransport. See Daemon · Transports for the trust posture of each.
  • Framing. NDJSON — one JSON value per line, separated by \n. Streams are opened in binary mode so Windows text-mode CRLF translation never mangles frame boundaries.
  • Envelope. Flat JSON object with a command discriminator on every request; {success, data | error} on every response. See Envelope and framing.
  • Commands. Seven verbs — ping, list, open, close, query, describe, invoke — covered in Commands.

The protocol in one round-trip

text example round-trip 
client → daemon
  {"command":"open","endpointUrl":"opc.tcp://plc.local:4840","config":{"securityPolicy":"http://opcfoundation.org/UA/SecurityPolicy#None"}}\n

daemon → client
  {"success":true,"data":{"sessionId":"a1b2c3...","reused":false}}\n

That is the entire wire — JSON, line, JSON, line. No length prefix, no XML envelopes, no Protobuf descriptor needed.

Constraints and gates

The daemon enforces a small set of envelope-level constraints. They are not configurable — they are part of the contract.

Constraint Value Why
Max frame size 65 536 bytes DoS gate — legitimate frames are < 2 KiB
Max per-connection buffer 1 048 576 bytes Backstop for misbehaving senders
Max concurrent connections 50 Per-process cap; configurable in code but not via CLI
Per-connection idle timeout 30 seconds Connection-level; not a per-request timeout

Frames that exceed any of these are rejected with payload_too_large or invalid_json and the connection is closed. See Reference · Exceptions.

Two RPC paths

The daemon exposes two dispatch paths into the underlying OPC UA client. Same command shape, different gating.

Path Use Gating
query Built-in OpcUaClientInterface methods Static whitelist (CommandHandler::ALLOWED_METHODS)
invoke Any method registered on the daemon's Client, including custom modules $client->hasMethod($name) + Wire type allowlist

ManagedClient routes the standard methods through query and any unknown method (via __call()) through invoke. Application code does not pick — the dispatcher does. See Commands.

Typed values cross the boundary

Every value crossing the IPC boundary is JSON-encoded by TypeSerializer (query path) or by the Wire layer (invoke path). The two formats are different in detail but share the same goal:

  • No unserialize() on the receiving side.
  • Explicit type tags for typed payloads.
  • Deterministic round-trip for the OPC UA value-object set: NodeId, Variant, DataValue, LocalizedText, QualifiedName, ReferenceDescription, …

See Type serialization for the per-type shapes.

What "authenticated" looks like on the wire

When the daemon requires an auth token, every request frame must carry an authToken field:

text authenticated frame 
{"command":"ping","authToken":"<shared-secret>"}

The daemon validates with hash_equals() (timing-safe) before dispatching. Failure responds with auth_failed and closes the connection. See Daemon · Authentication.

What is not in the IPC layer

  • Per-request authentication. The daemon-level shared secret is the only auth gate. Per-user authorisation has to live in your application code, not the IPC channel.
  • Asynchronous notifications. The IPC channel is request / response. Notifications from OPC UA subscriptions are surfaced via the daemon's auto-publish path (see Daemon · Auto-publish) — they do not stream back over the IPC channel.
  • Backpressure signalling. The 50-connection cap is a hard rejection, not a queue. Clients beyond the cap see a connection refused at the transport level.
  • Versioning. The wire format does not carry a version field. Compatibility is by behaviour: the flat {command, ...} envelope is the current shape and the only one the daemon accepts.

When the wire matters to you

In normal use, the IPC layer is invisible — ManagedClient and the daemon handle it for you. You reach for the wire-level docs when:

  • Debugging a stuck call. Run netcat against the socket and send a ping. See Recipes · Debugging with netcat.
  • Integrating from a non-PHP language. A Go or Python consumer speaks the same JSON envelope.
  • Writing a custom module that ships its own parameter shapes. See Extensibility · Custom param deserializer.
  • Operating a daemon at scale. Knowing the frame cap is the difference between diagnosing payload_too_large quickly and staring at logs.
Last updated · 2026-05-25
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